Twin-clutch transmissions consist of two partial transmissions, a group of gears and an engine clutch associated with each of them. Such a twin-clutch transmission is known, for example, from DE 35 46 454 A1.
Usually, the even-numbered gears are associated with one partial transmission and the odd-numbered gears with the other partial transmission so that in a normal, sequential gear-shifting operation, namely in going from an engaged old gear to the next higher or next lower target gear, one jumps between the partial transmissions. If the starting gear is associated with the first partial transmission, then first the engine clutch associated with the partial transmission of the new gear is opened, the new gear is engaged, and in a subsequent overlapping opening of the engine clutch associated with the partial transmission of the old gear and closing of the engine clutch associated with the partial transmission of the new gear, the new gear is engaged. The force transmission take place alternatingly via the first partial transmission with the first engine clutch and the first transmission input shaft and via the second partial transmission with the second engine clutch and the second transmission input shaft.
Twin-clutch transmissions are mostly automated, because manual operation of two engine clutches and gear-shifting with two gears engaged temporarily and at the same time would require a considerable mechanical effort.
From DE 101 09 662 A1 is known a control strategy for a twin-clutch transmission which is used in situations wherein multiple short-lasting changes in travel direction arise, for example when maneuvering from or in parking places or when rocking in snow. To avoid the many internal gear switching and clutching operations and at the same time minimize clutch wear, DE 101 09 662 A1 proposes, when a vehicle is in a travel direction-change mode in which frequent changes are made between forward and reverse travel, to keep a forward gear permanently engaged in one partial transmission and a reverse gear permanently engaged in the other partial transmission. The change in travel direction is then brought about by alternate opening of the engine clutch associated with one partial transmission and closing of the one associated with the other partial transmission. In the partial transmissions, the lowest of the reverse gears and the lowest of the forward gears are usually selected. Whether a vehicle is in a travel direction-change mode is determined either manually by actuation of a switch or automatically, a travel direction-change mode being assumed when within a certain predetermined time period several changes in travel direction are made to occur. The end of the travel direction-change mode is indicated, for example, by the brake pedal actuation exceeding a predetermined time interval or by the gas pedal actuation exceeding a certain brake pressure, a certain time interval or a predetermined pedal travel distance or the like.
Moreover, automated control of such a twin-clutch transmission often assumes pre-selection or early engagement of a starting gear when the vehicle is at a standstill and when the shift lever is in a non-driving position. In particular, this is intended to ensure a spontaneous reaction of the motor vehicle, particularly in combination with a parking brake that is actuated by the gear actuator. Moreover, particularly at low temperatures, pre-selection is often desirable to avoid switching noises, namely unpleasant noises during switching resulting from the drag torques in the clutch. Wet clutches have higher drag torques than do dry ones.
On the other hand, particularly under cold conditions when the viscosity of the clutch fluid (for example the transmission oil) is elevated, the drag torque can reach the magnitude of the creep torque of the motor vehicle so that it is possible for a vehicle with pre-selected starting gear to creep even though the clutch is not yet engaged, for example if the shift lever is in the neutral position (N) or park position (P).